As a reaction, which you use when you detect a teleportation within 20 meters, you can halt a creature teleporting in or teleporting out that occur within 20 meters. The creature must make a Will saving throw.
On a failure, its teleportation is delayed until the start of its next turn. At the start of its next turn, it repeats the Will saving throw or its teleportation is canceled.
On a critical failure, its teleportation is canceled.